There is a real problem with "Kayn, The Shadow Reaper", his design punishes you for good decisions.
I think Kayn as a champion design is really original, a lot of people might call him "edgy" or "emo" whatever the terms may be, but I believe his background and the whole idea that a human can hybrid because they possess a Darkin Scythe is fucking dope, but there comes a real problem with his in-game design and I'm not talking about his visuals, I'm talking about his skill-set and how he works as a champion.
Why am I forced to jungle as Kayn? You might be thinking, how are you forced to play a position, well i'll explain. You see Kayn has to collect orbs and the orbs he collects helps build towards his form, he can choose to be consumed by Rhaast or consume Rhaast and be the Shadow Assassin, or he can choose a form if he gets the form he "doesn't want" after 4 minutes. This is where the major problem comes from, lane Kayn is basically impossible if you want to achieve a certain form every game, because you're forced to lane against laners that are dynamic, some are melee in the mid lane although you want the assassin form, and some are ranged in the top lane although you want the Rhaast form. By making his skill-set work like this, you create a disadvantage from what should be an advantage, if i get enough orbs for either form whether it's me waiting 10 mins and getting 1 or 2 kills or poking up to 8 mins or so and getting enough orbs, I should be able to be rewarded with what form I desire not based on the RNG of my enemy's picks.
This is why Kayn is forced into the jungle because most games have a mixture (keyword "most") of ranged and melee champions and not being confined to a lane allows Kayn to choose whether he wants to attack melee or ranged champions, but why am I punished for choosing to punish the enemies mistakes? If i'm close to form and someone makes a mistake, I simply can't punish it because if I do, I'm now forced to wait 4 minutes for the form that I originally was going for and built for, why should I be forced to wait 4 minutes just because I made a good decision? It seems counter-intuitive to say the least. This is a backwards step from a competitive environment, I basically have to baby my enemies if they happen to be melee over range or vice versa and they get to do whatever they want without punishment because if I punish them I now have to wait 4 minutes before I get a form. Let's review why waiting 4 minutes for a form really is a terrible thing, pre-form Kayn gets majority of his damage from bonus ad and base damage, but he misses out on 70% bonus movement speed, up to 44% bonus magic damage converted based on the damage you did (post-mitigation) and also a HUGE ONE, the ability to actually combo your autos and other abilities with your W (Blade's Reach), the pre-form w locks out any autos or abilities, Shadow Assassin W allows you to cast it while being able to move allowing you to implement autos and a q after to create a massive difference in DPS, yes you can combine autos pre-form before the ability and after but the difference is blatant, there are clear differences in viability and versatility and same with Rhaast, in Rhaast you get a knock up for 1 second and the added 60% slow I believe it's for 1.5 seconds, so you have .5 seconds of slow after your knocked up and not to mention he gets like 45% of his ability damage converted to health and his ult/r (Umbral Trespass) now gives him a huge chunk of health back based on the enemies max health and his q (Reaping Slash) gets like standard 10-11% max health damage putting into account basic build AD, that can apply twice as well because you get damage going in and around so in theory you can do 22% ish max health damage with your q.
So as you can see there are clear advantages that both forms have that the pre-form lacks, but that being said both forms need specific builds to cater to their advantages, in 4 minutes a lot can happen in a game, a progression towards victory especially when you're inferior to the enemy, there are games where you can have items up on the enemy jungler or even laner and be weaker than them because their spikes are larger than yours even when they have a disadvantage by being levels behind or items behind because pre-form is meant to be less strong than the forms, the argument here isn't that pre-form needs a buff, it's that waiting 4 minutes for another form is a ridiculous addition to his skill-set. I'll propose something that's happen to me twice, the enemy team had all melee champs including the traditional ranged mid and bot, by definition this is a counter to me if I prepared for the assassin form, now why should I be forced into a disadvantage before the game even starts because the enemy team decided to pick melee champions over ranged champions and no this can't be compared with a composition that is tanky and a composition that is full damage and the composition that is full damage can just be built against with armor, etc because at least the composition with full damage can build penetration, as a Kayn player you can't do anything but wait the 4 minutes.
How do we make Kayn viable for lanes, more flexible in general and also not punished for making better decisions in the jungle and not letting people get away with over extending or coming into lane with no summoners just because they will give Kayn a form that forces him to wait 4 minutes, making him feels "forced" to opt on not punishing the enemies mistakes, which is anti-fun, counter-intuitive and ruins competitive integrity. We remove the limitation all together, it serves no real purpose, in theory yes it'd force you to change your routes and make the game more interesting, but all it does is force you to take punishment for punishing the enemy and never should this kind of game play exist in a game that is competitive, it's like putting limitations on how good a player can be, because i'm not allowed to be as good as I can be or I take punishment for it and that is anti-competitive. Remember at the end of the day majority of what League is, is decisions and for me to be opted out of decisions because a champion is designed to be punished for making what is the efficient decision, the appropriate one if you will, is just plain stupid...